Retroactive inhibition (Ri) is the decrement in retention attributable to interpolated learning. The most common type of RI study is one in which a particular variable is manipulated in the acquisition phase of the experiment, and the loss of v/ords from an initially learned list is examined as a function of the manipulation. The literature on RI has been reviewed a number of times in the last several decades (i.e., Swen son, 1041; Slamecka and Ceraso, 19G0; and Keppel, 1963). Slamecka and Ceraso make use of the following classification for independent variables which have been investigated: 1) degree of acquisition; 2) similarity of materials; 3)cxtrinsic factors; and 4)temporal effects